Hello and welcome.
In this video we will talk about periodic boundaries in Ansys Fluent Meshing Watertight Workflow.
In particular, we will learn about the different types of periodicities, how they are used
in simulations, and why it is important to include these boundaries in the meshing phase.
Also, we will explore how to extract the parameters needed for a manual set up of these boundaries.
So, let’s get started.
Periodic models can be defined as a series of repeating patterns.
For example, the rotor of an axial fan is a repetition of angular sectors that include
one of its blades.
In this case, we have a rotational periodicity since the patterns repeat around the same axis of rotation.
When analyzing turbulence development or heat transfer, we can find geometries with patterns
that repeat along a linear direction.
This is called translational periodicity.
Dividing the model into just one of these patterns reduces the mesh size and the computational cost.
Periodic boundary conditions impose that
‘what exits through one boundary is equal to what enters through the opposite boundary’.
This takes into account the adjacent parts of the model that are not simulated.
Note that periodic boundaries at the opposite sides of a geometry must be consistent
and have the same topology.
There are several advantages of including the periodic boundary setup during the meshing phase.
The process enforces the mesh on opposite periodic boundaries to be exactly the same,
or conformal.
Having a conformal mesh on periodic boundaries is the key point of setting them during the meshing phase,
instead of during the solver phase.
This removes the need to set up an interface, speeding up the setup of the periodic boundaries.
No interpolation of the solution is required, since mesh nodes would match on both sides,
ensuring accuracy of the solution compared to cases with inconsistent cells sizing
on the two periodic boundaries.
Also, the process will automatically set up the zones’ reference frame in the solver phase,
a feature especially useful when handling complex models.
Let’s now discuss some best practices that are good to keep in mind during the CAD preparation phase
before setting up the periodic boundaries in the meshing phase.
The Ansys Fluent Meshing Watertight Workflow gives the user two options to set up the periodic boundaries:
an automatic one and a manual one.
The Automatic method is extremely convenient and fast in setting up periodic boundaries,
as it automatically detects the angle and axis of rotation, or the translation vector
of the periodicity.
Keep in mind that there are some scenarioswhere it would not be able to detect the periodicity parameters:
First when the boundaries are non-planar surfaces.
Then, when the normal to the planar faces does not align with the tangent vector of the circular arc.
Also, if the rotational angle is smaller than 10 degrees, the periodicity parameters
may not be accurate.
In these cases, the manual method comes in handy.
When using this method, the user must specify the inputs for rotation or translation,
relative to the selected boundary.
It is good practice to extract these values for the rotation or translation during the CAD phase
and note them down.
They can also be used to verify that the automatic set up of the periodic boundaries is correct.
Let’s explore how to do so using Ansys SpaceClaim.
For a model with translational periodicity, we need to know the translational shift
along the three global axes.
In simple cases one can use the measure tool and select the periodic boundaries to estimate
the distance between them.
If the model is not aligned with any of the axes, then it is easier to pick the corner
on each face, note down the coordinates and estimate the shift along each axis.
In the case of rotational periodicity, the parameters to provide are
the periodicity angle, the rotation axis origin, and the rotation axis direction.
The recommended method to detect the rotation angle is to use the measure tool,
select one of the arcs and note down the arc angle.
This operation will show the axis of rotation.
Click on the axis and check the line properties.
Here the axis origin and direction will be listed.
Note down these values to use them later as inputs for the rotational periodicity.
When dealing with periodic models it is always recommended to align the axis of translation
or rotation with one of the axes of the global coordinates system.
Another important thing to consider is that only one set of periodic boundaries is supported
when using the watertight workflow.
Hence, in models with multiple regions that are aligned and have same periodicity,
it is suggested to group all the periodic faces on each side under a common named selection.
Let’s summarize what we have discussed in the video.
There are two types of periodicities: translational and rotational.
Setting up the periodic boundaries during the mesh phase enforces conformal mesh on both sides,
increasing the accuracy of the results, improving the computational speed,
and reducing the time to set up the model during the solver stage.
In Fluent Meshing Watertight workflow, we can set up periodic boundaries automatically or manually.
In particular cases, the automatic method would not detect the input parameters
and the manual method should be used instead.
As a best practice, it is suggested to extract the needed inputs from the geometry
before moving to the meshing stage.
Remember that only one set of periodic boundaries is supported,
hence it is convenient to group all the faces on one side of the model under the same label
and do the same on the opposite side.
With that we have come to the end of our video.